In portable electronic devices it is desirable to have a rechargeable battery with a high energy density, to extend operation life and reduce weight.
Several rechargeable battery chemistries exist, such as those based on Nickel Metal Hydride (NiMH) and Lithium Ions (LiIon). These chemistries have differing advantages and disadvantages:
Compared with NiMH batteries, Lithium Ion batteries suffer less from memory effects (where repeated partial discharging reduces capacity), and their capacity is also less temperature dependent. Lithium Ion batteries also exhibit a lower self-discharge rate, making storage easier.
However, Lithium Ion batteries take up to twice as long to recharge as NiMH batteries, and the procedure required to stably recharge Lithium Ion batteries is more proscriptive.
A Lithium Ion battery often comprises a number of cells, but can comprise just one. The cell consists of a carbon-based negative electrode and a lithium transition metal oxide positive electrode. The basic electrochemistry of a Lithium Ion cell involves the transfer of lithium ions between these two insertion electrodes.
Upon charging, lithium ions are extracted from the positive electrode material and inserted into the negative electrode material. Upon discharging, the reverse process takes place.
If charge and discharge currents and battery temperature are properly controlled, Lithium Ion batteries are safe. However, overcharging can convert the lithium oxide into metallic lithium, which is potentially dangerous.
Due to this sensitivity to charging conditions, battery manufacturers have developed ‘intelligent’ batteries. These incorporate sensors and electronics to monitor cell voltage, temperature and charge or discharge current.
Intelligent batteries are used in conjunction with intelligent charging systems to implement strict guidelines in charge procedures, wherein cells are charged to 4.20V/Cell with a tolerance of ±0.05V/cell.
FIG. 1 shows the cell voltage and charge current characteristics as the Lithium-ion cell passes through three stages of charging in a known procedure.
In stage 1, a constant charge is applied (dependant on cell size) until the cell voltage limit (4.2V/Cell) is reached.
At this point, the cell is 70-80% charged.
In stage 2, the cell voltage is maintained whilst the charge current starts to drop as full cell charge is approached. A full cell charge is attained once the charge current drops below a threshold percentile of the stage 1 charge current, or the charge current levels off. In stage 3 (storage), because Lithium-ion cells cannot accommodate overcharging, an occasional top-up charge is applied in lieu of a trickle charge, typically when the open terminal voltage drops below 4.05V/Cell.
However, as noted previously, temperature is an additional factor affecting the charging process. The charging temperature of Lithium Ion batteries is limited to approximately a +45° C. maximum. If this temperature is exceeded, the battery cannot charge and charging must be suspended.
An implementation of a prior art intelligent charging system to implement the above charging regime is shown in FIG. 2.
In FIG. 2, the battery pack 250 is connected at point 208 to the ‘battery’ pin of the Global Control/Audio/Power controller (GCAP) 240 or equivalent and at point 204 to both the radio frequency power amplifier (RFPA) 220 and the portable device 210 via P-Channel MOSFET switch 274.
When the portable device 210 is turned on, GCAP controls the P-Channel MOSFET switch 274 between the battery and output B+ 212 from its ‘Main_FET’ output pin 244.
When a charger 260 is connected and recognised by GCAP 240, the ‘Main_FET’ output 244 causes switch 274 to disconnect the battery from the portable device 210, and an external voltage is supplied to the device via the EXT_B+ pin 216 at point 201, through voltage protection circuits (not shown) and diode 272.
To charge, GCAP 240 controls the charging current using the P-Channel MOSFET 284 and an internal digital-to-analogue converter (not shown). The charger maintains a voltage of 1.4V plus the voltage on the battery feedback line connected to the BFDBK pin 242 of GCAP 240, within limits of 4.4V and 6.5V as defined in Table 1 below:
TABLE 1charging schemeBattery feedback, VVoltage Ext_B+Tolerance0.0-3.0 Volts4.4 Volts±5%3.0-5.1 VoltsBattery feedback + 1.4 V±5% 5.1+ Volts6.5 Volts+2%, −5%
The maximum mean charge current is limited to 1.5 Amps, whilst allowing charge spikes of up to 2.4 Amps to allow for RF power amplifier current requirements during transmission.
If the battery temperature exceeds approximately 45° C. then charging of the battery is suspended. However, due to the circuit arrangement of FIG. 2, drain on the battery from RF power amplifier 220 may continue. This has the potential to fully discharge the battery and consequently prevent the device operating.
The alternative is to force the device to switch off whenever the battery heats above 45° C., which may be extremely inconvenient.
The current solution known in the art is shown in FIG. 3. In FIG. 3, the RF power amplifier 220 derives its power from B+ 212 at point 301. Consequently, during charging RF power amplifier 220 takes power from the charger 260, with the result that it does not continue to drain the battery 250 if charging has been suspended due to battery overheat. This enables continuing operation of the device.
However, following the charging scheme for table 1, the charger voltage during stage 2 charging will be in the order of 1.4V+4.2V=5.6V. This is too high for some modern low-voltage RF power amplifiers, such as the 3V hetero-bipolar transistor (HBT), which has a maximum operating voltage of 4.8V.
As a consequence a new charging arrangement is required to accommodate modern low-voltage radio frequency power amplifiers.
The purpose of the present invention is to address the above requirement.